If you've got the folder with the *.xcf file already opened when you installed the thumbnailer, you'll need to refresh (ctrl+r) the view and then nautilus will start generating the thumbnails.If that doesn't work try restarting nautilus (alt+f2 and run: /usr/bin/nautilus -q && /usr/bin/nautilus --no-desktop).And also check if the file /usr/share/thumbnailers/xcf.thumbnailer exists.